A hydromechanical transmission is frequently used as a coupling between the traction wheels of a vehicle and its engine. In particular in heavy-duty construction equipment as well as in masstransit road vehicles it is frequently necessary to provide a transmission capable of producing a variable-speed output from a fixed-speed input so that the vehicle may be driven at different speeds by a prime mover such as a Diesel engine operating effectively at a more or less constant speed.
A typical such transmission can be seen in U.S. Pat. No. 3,489,036 issued Jan. 13, 1970 to Cockrell & Dorgan. In this drive a split-torque transmission has a planetary-gear train used to combine the speeds and torques of a hydrostatic unit of an input shaft. A second planetary-gear train is used to combine the speeds and torques of the input shaft and of the ring gear of the first planetary-gear train to drive the output shaft. Between the ring gear of the second planetary-gear train and the planet gears of the first train additional gearing is provided to drive the ring gear either in a direction opposite that of the planet gears or codirectionally therewith.
This transmission therefore, empolys two sets of planetary gears with a hydrostatic unit to achieve what is normally referred to as a two-range output, one of the ranges--normally the low range--also including a reverse-speed subrange. This assembly is, therefore, relatively effective but also rather complicated since amount of gearing necessary to achieve steplessly variable speed ratios across the two ranges is considerable. This general type of tranmission is therefore expensive to produce, failure-prone and difficult to service.